Process for the conversion of ethane to aromatic hydrocarbons

ABSTRACT

A process for producing aromatic hydrocarbons which comprises (a) contacting ethane with a dehydroaromatization aromatic catalyst which is comprised of 0.005 to 0.1% wt platinum, an amount of iron which is equal to or greater than the amount of the platinum, from 10 to 99.9% wt of an aluminosilicate, and a binder, and (b) separating methane, hydrogen, and C 2-5  hydrocarbons from the reaction products of step (a) to produce aromatic reaction products including benzene.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a divisional of a non-provisional applicationSer. No. 12/867,973 filed Oct. 7, 2010, now U.S. Pat. No. 8,692,043which is a national stage (§371) application of PCT/US2009/034364 filedAug. 27, 2009 which claims priority to U.S. Provisional PatentApplication No. 61/029,939 filed 20 Feb. 2008.

FIELD OF THE INVENTION

The present invention relates to a process for producing aromatichydrocarbons from ethane. More specifically, the invention relates to adehydroaromatization process for increasing the production of benzeneand/or total aromatics from ethane.

BACKGROUND OF THE INVENTION

There is a projected global shortage for benzene which is needed in themanufacture of key petrochemicals such as styrene, phenol, nylon andpolyurethanes, among others. Generally, benzene and other aromatichydrocarbons are obtained by separating a feedstock fraction which isrich in aromatic compounds, such as reformates produced through acatalytic reforming process and pyrolysis gasolines produced through anaphtha cracking process, from non-aromatic hydrocarbons using a solventextraction process.

In an effort to meet growing world demand for benzene and otheraromatics, various industrial and academic researchers have been workingfor several decades to develop catalysts and processes to make lightaromatics (benzene, toluene, xylenes, or BTX) from cost-advantaged,light paraffin (C₁-C₄) feeds. Prior-art catalysts devised for thisapplication usually contain an acidic zeolite material such as ZSM-5 andone or more metals such as Pt, Ga, Zn, Mo, etc. to provide adehydrogenation function. Aromatization of ethane and other loweralkanes is thermodynamically favored at high temperature and lowpressure without addition of hydrogen to the feed. Unfortunately, theseprocess conditions are also favorable for rapid catalyst deactivationdue to formation of undesirable surface coke deposits which block accessto the active sites.

For many hydrocarbon processing applications, one approach to reducingcatalyst performance decline rates due to coking is to increase thecatalyst metals loading in an effort to promote fasterhydrogenation/breakup of large coke precursor molecules on the surface.Another approach involves incorporation of additives such as phosphateor rare earths to moderate surface acidity and reduce coking rates underreaction conditions. These approaches are appropriate for processesfeaturing fixed or slowly-moving catalyst beds wherein the averagecatalyst particle residence time in the reactor zone betweenregenerations (coke burnoff steps) is relatively long (at least severaldays). For example, see U.S. Pat. Nos. 4,855,522 and 5,026,937, whichdescribe ZSM-5-type lower-alkane aromatization catalysts promoted withGa and additionally containing either a rare earth metal or aphosporus-containing alumina, respectively.

Yet another approach to circumvent this problem is to devise a loweralkane aromatization process in which the catalyst spends a relativelyshort time (less than a day) under reaction conditions before beingsubjected to coke burnoff and/or other treatment(s) aimed at restoringall or some of the original catalytic activity. An example of such aprocess is one featuring two or more parallel reactors containing fixedor stationary catalyst beds, with at least one reactor offline forcatalyst regeneration at any given time, while the other reactor(s)is/are processing the lower alkane feed under aromatization conditionsto make aromatics. Another example of such a process features afluidized catalyst bed, in which catalyst particles cycle rapidly andcontinuously between a reaction zone where aromatization takes place anda regeneration zone where the accumulated coke is burned off thecatalyst to restore activity. For example, U.S. Pat. No. 5,053,570describes a fluid-bed process for converting lower paraffin mixtures toaromatics.

Requirements for optimal catalyst performance in a process involving arelatively short period of catalyst exposure to reaction conditionsbetween each regeneration treatment, such as a fluidized-bed process,can differ from those of fixed- or moving-bed processes which requirelonger catalyst exposure time to reaction conditions betweenregeneration treatments. Specifically, in processes involving shortcatalyst exposure times, it is important that the catalyst not exhibitexcessive initial cracking or hydrogenolysis activity which couldconvert too much of the feedstock to undesirable, less-valuablebyproducts such as methane.

Certain metals such as Pt which are very suitable for catalyzing thedehydrogenation reactions that are essential for an alkanedehydroaromatization process can also, under certain circumstances,display undesirable hydrogenolysis activity that leads to excessiveproduction of methane from higher hydrocarbons. The inclusion of asecond, inert or less-active metal in a catalyst composition to helpsuppress the hydrogenolysis activity of the first, more-active metal isused in commercial scale catalytic naphtha reforming in which C₅-C₁₂paraffins and naphthenes are converted to aromatic compounds withcatalysts which are predominantly bimetallic and are supported onchloride-promoted alumina. As indicated in a catalytic naphtha reformingreview article by C. A. Querini in volume 6, pages 1-56 of theEncyclopedia of Catalysis (I. T. Horvath, ed.; published by John Wiley &Sons, Inc., Hoboken, N.J., USA, 2003), these catalysts typically containPt plus another metal such as Re (in sulfided form) or Sn. Among othereffects, these second metals can interact with the Pt to reducehydrogenolysis activity, thereby decreasing the rate of unwanted methaneformation.

These Pt/Re and Pt/Sn catalysts, supported on chloride-promoted alumina,are widely employed in fixed-bed (semi-regenerative) and moving-bed(continuous) naphtha reformers, respectively, and their compositions areoptimized for relatively long catalyst exposure times to reactionconditions between regeneration treatments. The average catalystparticle residence time in the reaction zone between regenerationtreatments ranges from a few days in moving bed reactors and up to 1 or2 years in fixed bed reactors. According to the article by Querinimentioned above, typical Pt and Sn levels in Pt/Sn naphtha reformingcatalysts are 0.3% wt each. Such catalysts, which usually lack astrongly acidic zeolite component, do not work well for lower alkanearomatization.

It would be advantageous to provide a light hydrocarbondehydroaromatization process which can be performed under conditionsthermodynamically favorable for light alkane aromatization as describedabove, which provides for relatively short catalyst exposure time toreaction conditions, wherein the average catalyst particle residencetime in the reaction zone between regeneration treatments may be from0.1 second to 30 minutes in a fluidized bed reactor and from a few hoursup to a week in moving bed and fixed bed reactors, and in which thecatalyst composition is optimized to reduce excessive initial productionof less-desirable byproducts such as methane.

SUMMARY OF THE INVENTION

The present invention provides a process for producing aromatichydrocarbons which comprises:

(a) contacting ethane with a dehydroaromatization catalyst wherein theethane contact time (the average residence time of a given ethanemolecule in the reaction zone under reaction conditions) is preferablyfrom 0.1 seconds to 1 minute, preferably 1 to 5 seconds, most preferablyat 550 to 730° C. and 0.01 to 1.0 MPa, said catalyst comprising:

-   -   (1) 0.005 to 0.1% wt (% by weight) platinum, basis the metal,        preferably 0.01 to 0.06% wt, most preferably 0.01 to 0.05% wt,    -   (2) an amount of iron which is equal to or greater than the        amount of the platinum but not more than 0.50% wt of the        catalyst, preferably not more than 0.20% wt of the catalyst,        most preferably not more than 0.10% wt of the catalyst, basis        the metal;    -   (3) 10 to 99.9% wt of an aluminosilicate, preferably a zeolite,        basis the aluminosilicate, preferably 30 to 99.9% wt, preferably        selected from the group consisting of ZSM-5, ZSM-11, ZSM-12,        ZSM-23, or ZSM-35, preferably converted to the H+ form,        preferably having a SiO₂/Al₂O₃ molar ratio of from 20:1 to 80:1,        and    -   (4) a binder, preferably selected from silica, alumina and        mixtures thereof;

(b) collecting the products from (a) and separating and recovering C₆₊aromatic hydrocarbons;

(c) optionally recovering methane and hydrogen; and

(d) optionally recycling C₂₋₅ hydrocarbons to (a).

The reactor system may comprise one or more reaction vessels, chambers,or zones, arranged in parallel or in series, in which contact betweenthe catalyst particles and the ethane-containing feed occurs. Thereactor vessel(s), chamber(s), or zone(s) may feature a fixed catalystbed (i.e., with parallel beds), a slowly-moving catalyst bed, or afluidized bed, In a preferred embodiment, a fluidized-bed reactor isused. The process is optimized to minimize the average catalyst particleresidence time while maintaining selectivity and conversion rate. Theaverage catalyst particle residence time is the average amount of timethat a catalyst particle is in the active reaction zone with ethanebetween regenerations.

Catalysts of the present invention—featuring lower levels ofdehydrogenation metal (preferably Pt) with potential cracking function,plus proper moderation of the dehydrogenation metal activity withappropriate amounts of a second, attenuating metal—are designed to limitinitial cracking activity without sacrificing the overall activity andaromatics selectivity required for commercially-viable production ratesof benzene and other aromatics.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a process for producing aromatic hydrocarbonswhich comprises bringing a hydrocarbon feedstock containing at least 50percent by weight of ethane or other C₂ hydrocarbons into contact with adehydroaromatization catalyst composition suitable for promoting thereaction of ethane to aromatic hydrocarbons such as benzene at atemperature of 550 to 730° C. and a pressure of 0.01 to 1.0 MPa. Theprimary desired products of the process of this invention are benzene,toluene and xylene.

The hydrocarbons in the feedstock may be ethane, ethylene or mixturesthereof. Preferably, the majority of the feedstock is ethane and from 0to 20 weight percent of the feedstock may be comprised of ethylene,preferably 5 to 10 weight percent. The feedstock may contain in additionup to 40 weight percent of other open chain hydrocarbons containingbetween 3 and 8 carbon atoms as coreactants. Specific examples of suchadditional coreactants are propane, propylene, n-butane, isobutane,n-butenes and isobutene. The hydrocarbon feedstock preferably containsat least 60 percent by weight of C₂ hydrocarbons, more preferably atleast 70 percent by weight. The reaction feed is often referred toherein as ethane for convenience but it is meant to include all of theother hydrocarbon materials referred to above if it is necessary ordesired for them to be present.

In a preferred embodiment, the reactor comprises a zone, vessel, orchamber containing catalyst particles through which theethane-containing feed flows and the reaction takes place. The reactorsystem may involve a fixed, moving, or fluidized catalyst bed. Thereaction products then flow out of the bed and are collected. Thereaction products are then separated and C₆₊ aromatic hydrocarbons arerecovered. Optionally, methane and hydrogen are recovered and optionallythe C₂₋₅ hydrocarbons are recycled to step (a).

A fixed bed reactor is a reactor in which the catalyst remainsstationary in the reactor and the catalyst particles are arranged in avessel, generally a vertical cylinder, with the reactants and productspassing through the stationary bed. In a fixed bed reactor the catalystparticles are held in place and do not move with respect to a fixedreference frame. The fixed bed reactor may be an adiabatic single bed, amulti-tube surrounded with heat exchange fluid or an adiabatic multi-bedwith internal heat exchange, among others. Fixed bed reactors are alsoreferred to as packed bed reactors. Fixed bed reactors provide excellentgas solids contacting. The fixed bed reactor configuration may includeat least two separate fixed beds in different zones so that at least onebed can be in active operation under reaction conditions while thecatalyst the other bed(s) is being regenerated.

In a moving bed catalytic reactor, gravity causes the catalyst particlesto flow while maintaining their relative positions to one another. Thebed moves with respect to the wall of the vessel in which it iscontained. The reactants may move through this bed with cocurrent,countercurrent or crossflow. Plug flow is the preferred mode. The movingbed offers the ability to withdraw catalyst particles continuously orintermittently so they can be regenerated outside the reactor andreintroduced into the circuit later on. Thus, there is an advantage tousing a moving bed when the catalyst has a short active life and can becontinuously regenerated. A moving bed reactor may consist of at leastone tray as well as supporting means for one or more catalyst beds. Thesupporting means may be permeable to gas and impermeable to catalystparticles.

A fluidized bed reactor is a type of reactor that may be used to carryout a variety of multiphase chemical reactions. In this type of areactor, a gas is passed through the particulate catalyst at high enoughvelocities to suspend the solid and cause it to behave as though it werea fluid. The catalyst particles may be supported by a porous plate. Thegas may be forced through the porous plate up through the solidmaterial. At lower gas velocities the solids remain in place as the gaspasses through the voids in the material. As the gas velocity isincreased, the reactor reaches the stage where the force of the fluid onthe solids is enough to balance the weight of the solid material andabove this velocity the contents of the reactor bed begin to expand andswirl around much like an agitated tank or boiling pot of water. Afluidized bed reactor is preferred for use in the present inventionbecause it provides uniform particle mixing, uniform temperaturegradients and the ability to operate the reactor in a continuous state.The catalyst leaves the reaction zone with the reaction products and isseparated therefrom in order to be regenerated before being recycled tothe reaction zone.

The ethane contact time may range from 0.1 second to 1 minute. Theethane contact time is the average amount of time that one molecule ofthe ethane feed is in the reaction zone. The preferred ethane contacttime is from 1 to 5 seconds. Longer ethane contact times are lessdesirable because they tend to allow for secondary reactions that leadto less-desirable byproducts such as methane and reduce selectivity tobenzene and/or total aromatics.

The catalyst comprises from 0.005 to 0.1% wt platinum, basis the metal.The platinum is highly active in terms of catalyzing thedehydroaromatization reaction and it is best if its concentration in thecatalyst not be more than 0.1% wt because otherwise too much methanewill be produced. In one embodiment, from 0.01 to 0.06% wt platinum isused and most preferably, from 0.01 to 0.05% wt of platinum is used.High performance is thus obtained with relatively low amounts of metalsin the catalyst.

An attenuating metal is an essential component of the catalyst of thepresent invention. The attenuating metal moderates the catalyticactivity of platinum so as to reduce the production of less-valuablemethane byproduct. The attenuating metal of the present invention isiron. For the present invention, the amount of the attenuating metal maybe equal to or greater than the amount of platinum, preferably not morethan 0.2% wt, more preferably not more than 0.10% wt of the attenuatingmetal is utilized in the catalyst because more than that can cause theoverall conversion to aromatics to become too low for commercial use.

The catalyst also comprises from 10 to 99.9% wt of one or morealuminosilicate materials, preferably from 30 to 99.9% wt, basis thealuminosilicate(s). The aluminosilicates preferably have a silicondioxide:aluminum trioxide molar ratio of from 20 to 80. Thealuminosilicates may preferably be zeolites having the MFI or MEL typestructure and may be ZSM-5, ZSM-8, ZSM-11, ZSM-12 or ZSM-35. The zeoliteor zeolite mixture is preferably converted to H⁺ form to providesufficient acidity to help catalyze the dehydroaromatization reaction.This can be accomplished by calcining the ammonium form of the zeolitein air at a temperature of at least 400° C.

The binder material serves the purpose of holding individual zeolitecrystal particles together to maintain an overall catalyst particle sizein the optimal range for fluidized-bed operation or to prevent excessivepressure drop in fixed or moving bed operation. The binder may beselected from the group consisting of alumina, silica, silica/alumina,various clay materials such as kaolin, or mixtures thereof. Preferably,amorphous inorganic oxides of gamma alumina, silica, silica/alumina or amixture thereof may be included. Most preferably, alumina and/or silicaare used as the binder material.

A platinum containing crystalline aluminosilicate, such as ZSM-5, may besynthesized by preparing the aluminosilicate containing the aluminum andsilicon in the framework, depositing platinum on the aluminosilicate andthen calcining the aluminosilicate. The attenuating metal may also beadded by the same procedure, either prior to, simultaneously with, orafter the addition of platinum. The metals may be added by any commonlyknown method for adding metals to such structures includingincorporation into the aluminosilicate framework during crystalsynthesis, or subsequent ion exchange into an already-synthesizedcrystal framework, or well as by various impregnation methods known tothose skilled in the art. The platinum and attenuating metal may beadded by the same or different methods.

In a preferred embodiment of the present invention an ethane feedstreamis introduced into the dehydroaromatization reactor. The feedstream thencomes into contact with the catalyst particles for the prescribed periodof time. The reaction products leave the reactor and are transferredinto a separator. The separator removes the aromatic products and theprincipal byproducts, methane and hydrogen, which preferably may berecovered, and also removes C₂₋₅ byproducts and unreacted ethane whichoptionally may be recycled to the dehydroaromatization reactor.

EXAMPLE

The following example is illustrative only and is not intended to limitthe scope of the invention.

Example 1

Catalysts A through D were prepared on samples of ZSM-5 zeolite powderCBV 3024E (30:1 molar SiO₂:Al₂O₃ ratio), available from ZeolystInternational. The powder samples were calcined in air up to 650° C. toremove residual moisture prior to use in catalyst preparation.

Metals were deposited on 25-50 gram samples of the above calcined ZSM-5powder substrate by first combining appropriate amounts of stocksolutions of tetraammine platinum nitrate and ammonium ferric oxalate,diluting this mixture with deionized water to a volume just sufficientto fill the pores of the substrate, and impregnating the substrate withthis solution at room temperature and atmospheric pressure. Impregnatedsamples were aged at room temperature for 2-3 hours and then driedovernight at 100° C.

Target platinum levels for catalysts A, B, and C were all 0.04% wt. Thetarget platinum level for catalyst D was 0.15% wt. Target iron levelsfor catalysts A, B, C, and D were 0, 0.04, 0.08, and 0.15% wt,respectively. The platinum and iron contents of catalyst D weredetermined by first calcining a sample of the catalyst at 550° C. todrive off residual moisture and render a loss on ignition (LOI)percentage. A known mass of untreated, ground catalyst D, corrected byLOI percentage, was digested using closed vessel microwave aciddigestion involving nitric, hydrochloric, and hydrofluoric acids. Thesolution was diluted to a known volume with deionized water, thenanalyzed for platinum and iron by directly coupled plasma emissionanalysis. The results of duplicate analyses were 0.160 and 0.165% wt forplatinum (average value 0.1625% wt) and 0.174 and 0.170% wt for iron(average value 0.172% wt). These results are expressed as weight percentbased on the weight of the 550° C.-calcined catalyst sample.

Catalysts made on the ZSM-5 substrate were first loaded into plasticbags and pressed at 20,000 psig for 2 min using an isostatic press. Theresulting “rock” was then crushed and sieved to obtain 2.5-8.5 mmparticles suitable for testing. For each performance test, a 15-cccharge of catalyst was loaded into a quartz tube (1.40 cm i.d.) andpositioned in a three-zone furnace connected to an automated gas flowsystem.

Prior to performance testing, all catalyst charges were pretreated insitu at atmospheric pressure as follows:

-   -   (a) calcination with air at 60 liters per hour (L/hr), during        which the reactor wall temperature was increased from 25 to        510° C. in 12 hrs, held at 510° C. for 4-8 hrs, then further        increased from 510 to 630° C. in 1 hr, then held at 630° C. for        30 min;    -   (b) nitrogen purge at 60 L/hr, 630° C. for 20 min;    -   (c) reduction with hydrogen at 60 L/hr, 630° C. for 30 min.

At the end of the pretreatment, 100% ethane feed was introduced at 15L/hr (1000 gas hourly space velocity-GHSV), atmospheric pressure, withthe reactor wall temperature maintained at 630° C. The total reactoroutlet stream was sampled and analyzed by an online gas chromatographysystem two minutes after ethane feed addition. Based on composition dataobtained from the gas chromatographic analysis, initial ethaneconversion and hydrocarbon product selectivities were computed accordingto the following formulas:ethane conversion,%=100×(100−% wt ethane in outlet stream)/(% wt ethanein feed)selectivity to hydrocarbon product Y(other than ethane)=100×(moles ofcarbon in amount of product Y generated)/(moles of carbon in amount ofethane reacted)For purposes of the selectivity calculation, C₉₊ aromatics were assumedto have an average molecular formula of C₁₀H₈ (naphthalene).

Analyzed platinum and tin levels and initial aromatization performancedata for Catalysts A-D, prepared and tested as described above, arepresented in Table 1. The data in Table 1 indicate that thelow-Pt/Fe/ZSM-5 catalysts B and C of the present invention (less than0.05% wt Pt, Fe level greater than the amount of Pt but not above 0.10%wt) provide better initial suppression of methane production and higherselectivity to benzene and total aromatics under ethane aromatizationconditions than catalysts A and D, in which the Pt/Fe levels falloutside the ranges of the present invention.

TABLE 1 Catalyst A B C D Target Pt Level, % wt 0.04 0.04 0.04 0.15Target Fe Level, % wt 0 0.04 0.08 0.15 Ethane conversion, % 60.39 58.0350.89 68.52 Selectivities, % (carbon basis) Methane 38.09 32.12 24.2441.25 Ethylene 6.52 7.1 7.95 3.58 Propylene 0.7 0.9 1.13 0.61 Propane0.7 0.86 1.3 0.45 C4 Hydrocarbons 0.14 0.16 0.21 0.14 C5 Hydrocarbons 00 0.01 0 Benzene 33.3 36.02 36.51 30.49 Toluene 16.32 16.79 18.8 14.05C8 Aromatics 2.75 2.6 3.63 2.44 C9+ Aromatics 1.49 3.45 6.22 6.99 TotalAromatics 53.85 58.85 65.16 53.97

What is claimed is:
 1. A catalyst comprising: (a) 0.005 to 0.1% wtplatinum, basis the metal, (b) an amount iron which is equal to orgreater than the amount of the platinum, basis the metal, but not morethan 0.2% wt of the catalyst; (c) 10 to 99.9% wt of an aluminosilicate,and (d a binder.
 2. The catalyst of claim 1 wherein the amount ofplatinum is from 0.01 to 0.06% wt.
 3. The catalyst of claim 1 whereinthe amount of platinum is from 0.01 to 0.05% wt.
 4. The catalyst ofclaim 1 wherein the catalyst comprises not more than 0.10% wt of iron.5. The catalyst of claim 1 wherein the amount of the aluminosilicate isfrom 30 to 99.9% wt and has a silicon dioxide:aluminum trioxide molarratio of from 20 to
 80. 6. The catalyst of claim 1 wherein thealuminosilicate has a silicon dioxide:aluminum trioxide molar ratio offrom 20 to
 80. 7. A catalyst comprising: (a) 0.005 to 0.06% wt platinum,basis the metal, (b) an amount iron which is equal to or greater thanthe amount of the platinum, basis the metal, but not more than 0.5% wtof the catalyst; (c) 10 to 99.9% wt of an aluminosilicate, and (d abinder.
 8. The catalyst of claim 7 wherein the amount of platinum isfrom 0.01 to 0.05% wt.
 9. The catalyst of claim 7 wherein the catalystcomprises not more than 0.20% wt of iron.
 10. The catalyst of claim 7wherein the catalyst comprises not more than 0.10% wt of iron.
 11. Thecatalyst of claim 7 wherein the amount of the aluminosilicate is from 30to 99.9% wt and has a silicon dioxide:aluminum trioxide molar ratio offrom 20 to
 80. 12. The catalyst of claim 7 wherein the aluminosilicatehas a silicon dioxide:aluminum trioxide molar ratio of from 20 to 80.